The first experiments claiming to see nonlinear instability in viscoelastic channel flow were performed by Arratia and colleagues [1]}, [2]}, [3]}. Finite amplitude perturbations were induced by an array of obstacles placed upstream, with the number of obstacles serving as a measure of amplitude. Based on measurements taken further downstream, far away from the initial disturbances, they conclude the existence of a subcritical nonlinear instability that persists down to \(Wi \approx 5.4\) . With the caveats that their channel had a square cross-section and FENE-P is an approximation, their results are encouragingly comparable to the 2D channel prediction made here of \(Wi_{sn}=7.5\) . In Figure S1 of the supplementary material to [1]}, the authors indicate the boundary to the observed instability in a \(Wi\)  vs. perturbation amplitude plane, essentially matching our predictions for the threshold of nonlinear instability given by the \(Re = 0\)  lower branch (shown in the middle-right panel of Figure REF )That the unstable region in our case is bounded by the branch is an immediate consequence of the discussion in Section REF : Once the threshold amplitude of a lower branch edge state is reached, solutions continue to grow..
However, in later proceedings, the authors claim that the unstable flow remains time dependent with features reminiscent of ET [2]}, [3]}, as opposed to the upper branch travelling wave scenario described here.
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